1. Field of the Invention
The present invention generally relates to a system for dissipating heat from a power module. More specifically, the invention relates to a system including a liquid cooled thermal stack for dissipating heat from a power module.
2. Description of Related Art
In high power electronic applications, such as electrical vehicle applications, a significant amount of heat is generated in a semiconductor device that controls the switching of power. The heat adversely affects the performance and reliability of the device by causing the device to overheat. When the device overheats, the junction temperature rises to a level where the device can fail to function. In addition, the devices and interconnects may also fail due to thermal expansion effects causing solder joint cracking. Therefore, it is advantageous to maximize in the device the capability to dissipate heat and to minimize the effects of thermal expansion.
One approach, as seen in FIG. 1, has been to use a direct bond copper (DBC) substrate. One example is illustrated by power module 10. The power module 10 includes a die 12, a DBC substrate 14, a heat spreader 20, and water 30 for cooling. The die 12 is attached to the DBC substrate 14 by a solder layer 16. A wire bond 26 attaches the die 12 to a bond pad 28 on the DBC substrate 14. The DBC substrate includes three layers, a top copper layer 15, followed by a middle aluminum nitride layer 17, and a bottom, third copper layer 19. Provided as such, the DBC substrate 14 provides a solderable, dielectric substrate for the die 12. In addition, the aluminum nitride layer 17 is dielectric with a coefficient of thermal expansion (CTE) closely matched to the silicon of the die 12.
The DBC substrate 14 is attached to the heat spreader 20 by the solder layer 18. Made of copper, the heat spreader 20 is attached to a layer of thermal grease 22 to a cold plate 24.
Fluid 30 is directed to flow across the copper cold plate 24 to transport the heat away from the power module 10, the fluid 30 is directed by a channel 32 defined by a first wall 34 and a second wall 36. An aperture 35 is formed in the first wall 34 of the channel 32 and the cold plate 24 is attached to the first wall 34 over the aperture 35. Provided with the aperture 35, the first wall 34 allows the water 30 to directly contact the cold plate 24 and dissipate heat. To seal the water 30 in the channel 32 a gasket 38 is provided between the first wall 34 and the cold plate 24.
Unfortunately, the DBC substrate 14 is not optimized for sinking heat from the die and may not provide optimal reliability. For example, the solder and thermal grease interfaces 18 and 22 may increase the thermal resistance of the thermal stack. In addition, stress due to thermal expansion mismatch of the copper with die 12 will be concentrated at the solder interfaces, which may result in failures in the solder. The advantage in using the DBC substrate includes using the substrate to support the electronic circuit since it has dielectric properties. Disadvantages of DBC include cost, low thermal conductivity, and difficulty of manufacturability.
In view of the above, it is apparent that there exists a need for an improved system for providing thermal dissipation of heat from semiconductor dies in high power electronic applications.